exhaust performance

[ben lee]

i know the manifold has a lot of effect on the power of the engine is there an ideal bore size of exhaust for a certain bhp?

on my 1.6 16v (k4m) hoping to be around 130-135bhp with my new mods. the original system was 40.5mm when 110bhp with a pre cat in the manifold then a cat.

now i will be running a 4-1 manifold into a 51mm downpipe/decat then reduced into a 40.5mm straight through centre then the backbox.

will the 40.5mm be to small? would going from 51mm to 40.5mm cause any problems?


what bore size is a 172 cat back system?

 

[singlespeed]

The RS133 has the cat on the end of the 4-1 manifold and uses 50mm O/D from the cat backwards.

 

[ben lee]

thanks singlespeed going to get a 51mm system to keep it all the same bore.